The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
The present invention generally relates to an apparatus and method for improving target track quality estimation in a passive sonar system.
(2) Description of the Prior Art
Conventional sonar systems estimate precision tracker performance qualitatively by generating a track quality indicator (xe2x80x9cTQIxe2x80x9d). There are four possible states for track quality (i.e., four TQI states): lost track, uncertain track, low signal-to-noise ratio (xe2x80x9cSNRxe2x80x9d), and strong track. The lost track state occurs when the signal strength of the tracker falls below a predetermined minimal SNR. The uncertain track state occurs when the estimated SNR is below the designed tracker threshold and/or the smoothed tracker error residual becomes large due to target dynamics. The low signal-to-noise ratio (SNR) track is declared when the estimated SNR is near the tracker design threshold and there are no significant biases induced by target dynamics. The strong track state occurs when the target SNR is significantly greater than the tracker design threshold and there are no biases induced by target dynamics.
The TQI provides information to the sonar operator and automated sonar data processing algorithms so that the status of all trackers can be monitored. Such a feature is especially important in sonar systems currently in use wherein the number of trackers is relatively large. When a lost track indication occurs, the sonar operator typically drops the track or reinitializes the tracker. An uncertain track indication occurs when the tracker is tracking, but the SNR is low and sonar operator intervention may be required. A strong track indication occurs when the tracker loop is locked and sonar operator intervention is unnecessary.
One significant disadvantage of many prior art TQI algorithms currently in use is that they exhibit instability when the tracker is near the boundary between TQI states. Such instability degrades track quality estimation.
A first object of the present invention is providing a method for indicating the quality of tracks in a passive sonar system.
Another object of the invention is providing such a method that is not overly sensitive to track status changes.
Accordingly, the present invention is directed to a method and apparatus for calculating a track quality indicator from a plurality of tracker amplitude estimates. In one embodiment, the method utilizes the track quality indicator at the previous time increment. The track quality indicator is assigned as a lost track based on a first lost track threshold if the track quality indicator at the previous time increment indicated a lost track and based on a second lost track threshold if the track quality indicator at the previous time increment indicated a different status. The track quality indicator is assigned as an uncertain track based on a first uncertain track threshold if the track quality indicator at the previous time increment indicated a lost track or an uncertain track and based on a second uncertain track threshold if the track quality indicator at the previous time increment indicated a different status. The track quality indicator is assigned as a low signal to noise ratio (SNR) track based on a first low SNR track threshold if the track quality indicator at the previous time increment indicated a strong track and based on a second low SNR track threshold if the track quality indicator at the previous time increment indicated a different status. The track quality indicator is assigned as a strong track if another track quality indicator is not assigned. The method continues to the next time increment after the track quality indicator is assigned.